Touch-ups to DataStruct

Makefile

@@ -45,7 +45,7 @@ latex/%.dvi: latex/%.tex
 	cd latex ; latex $* ; latex $*
 
 latex/%.pdf: latex/%.dvi
-	cd latex ; pdflatex $* ; pdflatex $*
+	cd latex ; pdflatex $*
 
 html: Makefile $(VS) src/toc.html
 	mkdir -p html

src/DataStruct.v

@@ -830,7 +830,7 @@ Qed.
 
    Inductive types are often the most pleasant to work with, after someone has spent the time implementing some basic library functions for them, using fancy [match] annotations.  Many aspects of Coq's logic and tactic support are specialized to deal with inductive types, and you may miss out if you use alternate encodings.
 
-   Recursive types usually involve much less initial effort, but they can be less convenient to use with proof automation.  For instance, the [simpl] tactic (which is among the ingredients in [crush]) will sometimes be overzealous in simplifying uses of functions over recursive types.  Consider a function [replace] of type [forall A, filist A n -> fin n -> A -> filist A n], such that [replace l f x] should substitute [x] for the element in position [f] of [l].  A call to [replace] on a variable [l] of type [filist A (S n)] would probably be simplified to an explicit pair, even though we know nothing about the structure of [l] beyond its type.  In a proof involving many recursive types, this kind of unhelpful "simplification" can lead to rapid bloat in the sizes of subgoals.
+   Recursive types usually involve much less initial effort, but they can be less convenient to use with proof automation.  For instance, the [simpl] tactic (which is among the ingredients in [crush]) will sometimes be overzealous in simplifying uses of functions over recursive types.  Consider a call [get l f], where variable [l] has type [filist A (S n)].  This expression would be simplified to an explicit pair, even though we know nothing about the structure of [l] beyond its type.  In a proof involving many recursive types, this kind of unhelpful "simplification" can lead to rapid bloat in the sizes of subgoals.
 
    Another disadvantage of recursive types is that they only apply to type families whose indices determine their "skeletons."  This is not true for all data structures; a good counterexample comes from the richly-typed programming language syntax types we have used several times so far.  The fact that a piece of syntax has type [Nat] tells us nothing about the tree structure of that syntax.